This invention relates to a solder feeding system of the type whereby pieces of solder are supplied to a soldering bath to maintain the level of the liquid solder in the bath.
A solder feeding system is required to automatically supply the same amount of solder as consumed for soldering to a bath in which articles, such as printed circuit boards, are successively soldered by the liquid solder. The liquid solder level should be prevented from fluctuating out of a tolerable range. For example, the fluctuation of the liquid solder level is desirably maintained within a range of -0.3 mm to +0.3 mm, to achieve precise soldering of finely printed circuit boards. The system is also needed to supply solder without bringing about a sudden temperature-drop in the soldering bath to avoid deterioration in soldering quality. As disclosed in Japanese Patent Publication (Tokyo Koho Sho, No. 55-19173), a known system comprises a solder bar of about 4 cm.times.5 cm.times.50 cm suspended by a belt and a pulley rotated by a motor to drive the belt, whereby the motor is rotated in one direction to dip the solder bar in the soldering bath when the liquid solder level drops below a preselected lower limit and is reversed to pull up the solder bar from the bath when the solder level exceeds a predetermined upper limit. However, for use with the soldering bath in which finely printed circuit boards are soldered, the solder bar is not only too large in size to reduce the fluctuation of the liquid solder level within a desirable range but also it is too large in thermal capacity to prevent the temperature of the liquid solder from suddenly dropping below an allowable limit. Besides, the solder bar is heavy and somewhat inconvenient to handle.
It is an object of the invention to provide a solder feeding system having such pieces of solder that are suitable for use with a soldering bath in which finely printed circuit boards are soldered.
It is another object of the invention to provide a solder feeding system having such pieces of solder that are supplied to a soldering bath without fluctuating the liquid solder level beyond a tolerable limit.
It is a further object of the invention to provide a solder feeding system having such pieces of solder that are supplied to a soldering bath without lowering the temperature of liquid solder in the bath below a tolerable limit.
The feeding system of the present invention comprises solder balls in the form of pinballs having a diameter of from 5 mm to 50 mm, preferably 10 mm to 30 mm, and an apparatus for automatically feeding a certain number of the solder balls to a soldering bath when the liquid solder level drops below a preselected limit, the number of solder balls being determined so as to raise the level of molten solder in the bath by an amount which keeps the level at or slightly less than the allowable range of fluctuation.
The solder ball is manufactured by various methods. An ingot of solder alloy is made molten and poured into a mold forming a plurality of spherical hollows to produce solder balls. A solder wire is shaped to the solder balls of a diameter slightly larger than that of the solder wire by cold or hot forging. Otherwise, the solder wire is continuously converted to the balls by cold or hot rolling. In general, the casted ball needs no additional working process to improve its rollability.
The feeding apparatus comprises a hopper disposed over a soldering bath for storing and arranging the solder balls in a plurality of lines, a discharge means mounted in the hopper for adding a preselected number of the solder balls into the soldering bath from the hopper, and a level control means for detecting the liquid solder level in the soldering bath and actuating the discharge means. The number of solder balls added each time from the discharge means is adjusted to raise the liquid solder level in the bath by a height less than a distance between the preselected upper and lower limits, for example, 0.6 mm of -0.3 mm to +0.3 mm. The narrow distance between the upper and lower limits reduces the number or amount of solder balls to be added at one time to the soldering bath such that due to the low thermal capacity of the solder balls a sudden temperature drop below a tolerable limit in the bath does not occur.
The advantages offered by the present invention are mainly that the level and the temperature of the liquid solder in the soldering bath can be kept substantially constant while the solder balls are supplied to the bath. Solder balls are easy to handle and are automatically supplied not only to a single bath but also to a plurality of baths, even if every bath differs from the other in capacity or operation. All in all, articles such as finely printed circuit boards can be uniformly soldered in the soldering bath.
The above and other objects and advantages of the invention will become more apparent from the following description and accompanying drawings forming part of the invention.